Workers in Germany's powerful IG Metall union began a series of national "warning strikes" in support of a pay increase more than twice that offered by engineering and electrical employers on Sunday and expanded their protests on Monday to include thousands of union members.

Officials at IG Metall, Germany's largest trade union with close to 2.4 million members, said strikes lasting around an hour began Sunday and had taken place in Berlin, the Rhineland, in Bavaria and in Lower Saxony.

Staff picketed Monday at a Siemens factor in Duisburg while other protests were planned in Vreden, Bocholt, Krefeld and Recklinghausen.

Michael Sommer, chairman of the DGB labour umbrella body, called for "real pay increases" for workers and for the government to push through legislation providing for a minimum wage "not under 7.50 euros ($10) an hour."

"This marks the end of the period when wage restraint was needed," Sommer said in reference to Germany's steadily growing economy.

Martin Kannegiesser, the head of Germany's metals and engineering employers' association Gesamtmetall, was quoted as saying he wanted a swift pay deal to prevent more damaging temporary stoppages.

"All arguments have been exchanged and all the building blocks are on the table," Kannegiesser, told Focus magazine in an interview published on Saturday.

Unions demand 6.5 percent pay hike

The IG Metall strikes began immediately after midnight when a deadline ran out. Under German law the warning strikes commence only after a fixed negotiation period. The two parties had failed to reach agreement after four rounds of talks.

The union is demanding 6.5 percent more pay for the 3.4 million workers in the sectors it covers. Employers have offered 2.5 percent plus a one-off bonus of 0.5 percent.

Union deputy leader Berthold Huber warned of "massive and widespread strikes" and said the union was ready for protracted strike action in the weeks ahead.

"IG Metall is prepared to fight and will lead this battle," Huber said. "And they will conduct it so that it really hurts."

Among the companies affected in the first wave of strikes were Bosch, Osram and MAN.

Auto firms in strikers sights

Some 600 IG Metall members at DaimlerChrysler in the southwestern state of Baden-Württemberg put down their tools early on Monday and plan to again on Wednesday, when workers would also demonstrate at Porsche and Audi.

Hartmut Meine, head of the IG Metall negotiating team in Lower Saxony, was quoted as saying on Saturday that if no accord was sealed by May 11, the union would consider whether to launch full strike action instead of the temporary stoppages that typically last a couple of hours.

The fifth round of talks between employers and the union take place in Baden-Württemberg on Thursday.

The Verdi service sector union, Germany's second-largest, is currently engaged in a battle with Deutsche Telekom, the former monopoly telecommunications company, over plans to shift 50,000 workers to a separate company with lower pay and longer hours.

Warning strikes have taken place, and the union, which accepts the longer working hours but rejects the pay cuts, has announced further industrial action in the weeks ahead.